The present invention relates generally to electrochemical double layer capacitors, and more particularly to a high performance electrochemical double layer capacitor made with low-resistance carbon powder electrodes.
Double layer capacitors, also referred to as electrochemical double layer capacitors (EDLC), are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, because of their relatively low internal resistance, double layer capacitors can typically be charged and can, in turn, deliver stored energy at a higher power rating than rechargeable batteries.
Double layer capacitors may consist of two carbon electrodes that are isolated from electrical contact by a porous separator. Both the porous separator and the electrodes are immersed in an electrolyte solution, allowing ionic current (ionic flow) to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to an ionic) current from shorting the two carbon electrodes.
Coupled to the back of each of the two carbon electrodes is typically a current collecting plate. One purpose of the current collecting plates is to reduce ohmic losses, i.e., internal resistance, in the double layer capacitor.
Double layer capacitors store electrostatic energy in a polarized liquid layer that forms when an electrical potential exists between the two carbon electrodes immersed in an electrolyte (or electrolyte solution). When the electrical potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name “double layer” capacitor) by the polarization of electrolyte ions due to charge separation under the applied electrical potential, and also due to dipole orientation and alignment of electrolyte molecules over an entire surface of the electrodes.
Fabrication of double layer capacitors with carbon electrodes is described in U.S. Pat. No. 2,800,616 (Becker), and U.S. Pat. No. 3,648,126 (Boos et al.).
A major problem in many carbon-electrode capacitors, including electrochemical double layer capacitors with carbon electrodes, is that the performance of the carbon-electrode capacitor is often limited because of high internal resistance related to the carbon electrodes. This high internal resistance may be due to several factors, including high contact resistance of carbon-carbon contacts within the carbon electrodes, and further including high contact resistance of the electrode-current collector contacts. This high internal resistance translates to large ohmic losses in the carbon-electrode capacitor during charging and discharging of the carbon-electrode capacitor. These high ohmic losses further adversely affect, i.e., increase, a characteristic RC (resistance times capacitance) time constant of the capacitor and thus interfere with the carbon-electrode capacitor's ability to be efficiently charged and/or discharged in a short period of time.
There is thus a need in the art for systems and methods that lower the internal resistance within a carbon-electrode capacitor, and hence lower the characteristic RC time constant, of the carbon-electrode capacitors, as well as other improvements.
U.S. Pat. No. 5,907,472 to Farahmandi et al., the complete disclosure of which is incorporated herein by reference, discloses a multi-electrode double layer capacitor having aluminum-impregnated carbon cloth electrodes. The use of the aluminum-impregnated carbon cloth electrodes described therein results in an electrochemical double layer capacitor having a very low internal resistance.
U.S. patent application Ser. No. 09/569,679 of Nanjundiah et al., to which priority is claimed in the present patent document, and the complete disclosure of which is incorporated herein by reference, discloses an electrochemical double layer capacitor having low-resistance carbon powder electrodes.
There is also a continuing need for improved electrochemical double layer capacitors. Such improved electrochemical double layer capacitors need to deliver large amounts of useful energy at a very high power output, and very high energy density ratings within a relatively short period of time. Such improved electrochemical double layer capacitors should also have a relatively low internal resistance, and hence a relatively low characteristic RC time constant, and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is apparent that improvements are needed in the techniques and methods of fabricating electrochemical double layer capacitor electrodes so as to lower the internal resistance of the electrochemical double layer capacitor, and hence lower the characteristic RC time constant and maximize the operating voltage.
Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. Thus, improved techniques and methods are needed to lower the internal resistance of the electrodes used within an electrochemical double layer capacitor and increase the operating voltage.